Oral phenylbutyrate is only available as 500 mg tablets at this time. Patients will be instructed to swallow the 500 mg tablets at 8am, 2pm, & 8pm each day. This will be 14 tablets per dose in cohort 1, 20 tablets per dose in cohort 2, and 28 tablets per dose in cohort 3. The placebo for each cohort will be dosed similarly (14,20, or 28 tablets per dose). Cohorts 2 & 3 are likely to have difficulty with swallowing so many pills around meal times and so will be cautioned to eat 2 hrs before or 1 hr after taking the pills. The first 3 days on protocol will be in the PCRC, where the nurse will assist the patient with drug administration & timing around meals. Because there is a large sodium load with the drug--uup to 4.8 gm per day, the PCRC nutrition service will design a 2 gm sodium diet for the inpatient & outpatient days to control the total sodium intake to 7 gm per day (Average no salt added diet is 4 gm per day). Both placebo & drug patients will be on the limited sodium diet to maintain blinding. We will then liberalize the sodium during the washout period to adjust for absence of drug.)